1. Field of the Invention
The present invention generally relates to antenna devices and, more particularly, to an antenna device composed of a multi-element antenna, operated at a plurality of frequencies and provided with matching circuits adapted for reflection coefficients.
2. Description of the Related Art
FIG. 1 shows a construction of a conventional antenna device-disclosed, for example, in U.S. Pat. No. 5,828,348; this example is the case of a 4-element antenna operated at two frequencies, and matching circuits connected to the 4-element antenna are the same.
In FIG. 1, symbols 101a, 101b, 101c and 101d denote antenna elements, symbols 102a, 102b, 102c and 102d denote parasitic antenna elements, symbols 103a, 103b, 103c and 103d denote matching circuits connected respectively to the antenna elements 101a, 101b, 101c and 101d, symbols 104a and 104b denote divider/combiner circuits using double branch line circuits for dividing an inputted signal into two signals with a phase difference of 90 degrees, numeral 105 denotes a 180-degree divider/combiner circuit for dividing an inputted signal into two signals with a phase difference of 180 degrees, and numeral 106 denotes an input/output terminal.
FIG. 2 shows a cylindrical dielectric 30 on the surface of which an antenna portion composed of the antenna elements 101a, 101b, 101c, 101d and parasitic antenna elements 102a, 102b, 102c, 102d of FIG. 1 is provided. As shown in the figure, the antenna elements 101a, 101b, 101c and 101d are formed on the outer surface of the cylindrical dielectric 30, while the parasitic antenna elements 102a, 102b, 102c and 102d are formed on the inner surface of inside diameter of the cylindrical dielectric 30.
The operation of the antenna device will now be described.
A signal inputted to the input/output terminal 106 is divided by the 180-degree divider/combiner circuit 105 as signals having phases of 0 degree and xe2x88x92180 degrees. Thereafter, one of the signals is divided by the divider/combiner circuit 104a as signals having phases of 0 degree and xe2x88x9290 degrees, and the other is divided by the divider/combiner circuit 104b as signals having phases of xe2x88x92180 degrees and xe2x88x92270 degrees. At two operating frequencies f1 and f2, the 180-degree divider/combiner circuit 105 realizes a phase distribution of 0 degree and xe2x88x92180 degrees, while the divider/combiner circuits 104a and 104b realize a phase distribution of 0 degree and xe2x88x9290 degrees.
In order to realize matching for each of the antenna elements 101a, 101b, 101c and 101d at the two frequencies f1 and f2, a scattering matrix of the antenna is determined empirically or by calculation, and reflection coefficients in operation are determined using excitation amplitude and excitation phase. In this example, due to symmetry of the scattering matrix of the antenna and symmetry of the excitation phase, the reflection coefficients of the antenna elements 101a, 101b, 101c and 101d are equal. Accordingly, the matching circuits 103a, 103b, 103c and 103d connected respectively to the antenna elements 101a, 101b, 101c and 101d are the same.
The entire divider/combiner circuit composed of the 180-degree divider/combiner circuit 105 and the divider/combiner circuits 104a and 104b is large in size, as shown in FIG. 1. Thus, as shown in FIG. 2, the entire divider/combiner circuit cannot be formed on the cylindrical dielectric 30, and, therefore, only the antenna portion composed of the antenna elements 101a, 101b, 101c, 101d and the parasitic antenna elements 102a, 102b, 102c, 102d is formed on the cylindrical dielectric 30.
FIG. 3 shows a conventional small-type divider/combiner circuit constructed by combining T branches with lines of unequal lengths. In the figure, symbols 107a, 107b, 107c and 107d denote excitation terminals, numeral 108 denotes an input/output terminal, and symbols 109a, 109b, 109c and 109d denote lines having lengths according to desired excitation phases. The lengths of the lines are such that 109a less than 109b less than 109c less than 109d, and the excitation phase is progressively delayed in the order of 107a, 107b, 107c and 107d. 
In the small-type divider/combiner circuit composed of T branches and lines of unequal lengths shown in FIG. 3, where the antenna device is operated at a plurality of frequencies, it is difficult to realize excitation with progressive phase shifts of a predetermined angle at all the frequencies. For example, where the lines 109a, 109b, 109c and 109d are set for excitation with symmetric phases by providing progressive phase shifts of 90 degrees at a frequency f1, the progressive phase shifts of 90 degrees cannot be achieved but asymmetric excitation results at a frequency f2 different from the frequency f1, and, therefore, the reflection coefficients at the antenna elements 101a, 101b, 101c and 101d are not equal to each other.
Since the conventional antenna devices are constituted as described above, there is the problem that the 180-degree divider/combiner circuit 105 and the divider/combiner circuits 104a and 104b for excitation with progressive phase shifts of a predetermined angle at operational frequencies f1 and f2 become very large, as shown in FIG. 1.
Therefore, where the antenna elements 101a, 101b, 101c, 101d, the matching circuits 103a, 103b, 103c, 103d, the divider/combiner circuits 104a, 104b and the 180-degree divider/combiner circuit 105 shown in FIG. 1 are formed on respective substrates and the substrates are connected to each other by cables or other connecting mechanisms, there is the problem that the antenna device as a whole becomes very large.
Besides, in the case of the small-type divider/combiner circuit composed of the T branches and the lines of unequal lengths shown in FIG. 3, there is a problem that it is difficult to achieve excitation with progressive phase shifts of a predetermined angle at both the operational frequencies f1 and f2, so that the reflection coefficients at the antenna elements 101a, 101b, 101c and 101d are not equal to each other, so that matching cannot be attained.
Accordingly, a general object of the present invention is to provide an antenna device in which the aforementioned disadvantages are eliminated.
Another and more specific object is to provide an antenna device which realizes smallness in size by using a small-type divider/combiner circuit such as the one shown in FIG. 3 and it is possible to attain matching of a multi-element antenna at a plurality of operational frequencies by connecting different matching circuits respectively to the antenna elements 101a, 101b, 101c and 101d. 
Still another object of the invention is to obtain an antenna device which is reduced in overall size by integrally forming antenna elements, matching circuits and divider/combiner circuits on a cylindrical dielectric.
According to the present invention, there is provided an antenna device comprising a plurality of antenna elements operated at a plurality of frequencies, a divider/combiner circuit for exciting the plurality of antenna elements at desired phases, and matching circuits each connected to the antenna element at one end and connected to the divider/combiner circuit at the other end, the matching circuits corresponding to reflection coefficients of the antenna elements determined by taking into account the coupling between the antenna elements occurring when the antenna elements are excited with corresponding excitation amplitudes and excitation phases at each of the frequencies.
This is effective in that it is possible to attain impedance matching of each of the antenna elements at the plurality of operational frequencies.
According to the present invention, there is provided an antenna device wherein the divider/combiner circuit is constructed by combining T branches with different-length lines.
This is effective in that the antenna device can be made smaller in size.
According to the present invention, there is provided an antenna device wherein branch line circuits are used as the divider/combiner circuit.
This is effective in that the antenna device can be made smaller in size, and designing of the matching circuits can be easily realized.
According to the present invention, there is provided an antenna device wherein the plurality of antenna elements, the divider/combiner circuit and the matching circuits are integrally formed on a surface of a cylindrical dielectric.
This is effective in that the antenna device can be made smaller in size.
According to the present invention, there is provided an antenna device wherein parasitic antenna elements are disposed in the vicinity of said antenna elements.
This is effective in that a desired radiation pattern can be obtained from the antenna device.
According to the present invention, there is provided an antenna device wherein the plurality of antenna elements, the divider/combiner circuit and the matching circuits are integrally formed on a surface of a first cylindrical dielectric and the parasitic antenna elements are integrally formed on a surface of a second cylindrical dielectric different in inside diameter from the first cylindrical dielectric.
This is effective in that the antenna device can be made smaller in size.